1. Field of the Invention
The present invention relates to a toner conveying roll and a developing apparatus used with an image forming apparatus of electrophotographic or electrostatic type such as a copying machine, a printer and the like.
2. Related Background Art
In copying machines and printers, it is practical that, after an electrostatic image on an image bearing member is developed by a developing apparatus, the developed image on the image bearing member is transferred onto a paper sheet. In such a developing apparatus, by rotating a developing sleeve as a developer (toner) bearing member bearing the toner, the toner is conveyed to a developing station where the developing sleeve is opposed to the image bearing member (photosensitive drum), thereby developing the electrostatic latent image on the photosensitive drum.
In recent years, since an improvement in a resolving power and sharpness of an image has been requested, it is important to form a thin toner layer on the developing sleeve, and, thus, development of method and apparatus for forming the thin toner layer is inevitable. Several techniques regarding this improvement have been proposed.
For example, as disclosed in Japanese Patent Application Laid-Open No. 54-43038, an elastic blade made of runner or metal is urged against a developing sleeve, and, by regulating toner by passing the toner through a nip between the elastic blade and the developing sleeve, a thin toner layer is formed on the developing sleeve and adequate tribo is applied at the nip.
In this case, when nonmagnetic toner is regulated by the elastic blade, an additional toner supplying member for supplying the toner onto the developing sleeve is required. The reason is that, in case of magnetic toner, although the toner can be supplied onto the developing sleeve by a magnetic force of a magnet disposed within the developing sleeve, in case of the non-magnetic toner, the toner cannot be supplied by the magnetic force. In order to supply the toner to the developing sleeve, in the past, techniques in which a roller member formed from a solid rubber material, brush material or foam rubber material is urged against the developing sleeve have been proposed.
The roller member must serve to peel or scrape the toner (not used for development; referred to as "non-development toner" hereinafter) remaining on the developing sleeve, as well as the supply of the toner to the developing sleeve. If such functions are inadequate, the non-development toner is mixed with toner newly supplied in a developing container and the mixed toner exists on the developing sleeve, with the result that, when the toner is next sent to the developing station (where the developing sleeve is opposed to the photosensitive drum) through the elastic blade regulating portion, developing ability is changed due to the difference of toner (new toner and mixed toner), thereby generating a so-called "ghost image".
Accordingly, regarding the roller member, both the toner supplying function and the toner scraping function are required.
First of all, considering the toner supplying function, it is difficult to provide adequate toner conveying ability using a solid material because of surface smoothness of the material, with the result that adequate toner cannot be supplied to the developing sleeve. A brush material has excellent toner supplying ability, but, since dislodging and/or shearing of brush fibers occurs or the brush fibers fall off or are laterally, the brush material is not preferable. To the contrary, since a foam rubber material has many foam bubbles in its surface, the toner conveying ability is greatly improved in comparison with the solid rubber material. Thus, the foam rubber material is suitable as the toner supply roller for supplying the toner to the developing sleeve, and, thus, the foam rubber material has widely been used.
Next, considering the toner scraping function, a method for mechanically scraping the toner by urging a foam elastic roller against an abut portion and a method for electrostatically scraping the toner from the abut portion by applying bias to the foam elastic roller as disclosed in Japanese Patent Application Laid-Open No. 2-191974 are known.
However, since a higher quality image has been requested in recent years, the toner used in the developing apparatus must have smaller particle diameter. Such a toner has a tendency that it is more difficult to be scraped in comparison with the toner having normal particle diameter. Accordingly, in order to prevent occurrence of the ghost image, it is required that the scraping ability is further improved. To this end, it is considered that a mechanical force is further added by using a foam elastic roller having the adequate toner supplying ability and by increasing a penetrating amount of the roller to the developing sleeve, an abut width (referred to as "nip width") between the roller and the developing sleeve or the number of revolutions of the roller.
However, if such a method is used, stress on the toner and the developing sleeve is increased, with the result that, due to deterioration of toner, poor charging, toner fusion and solidification on the developing sleeve, damage of the surface of the developing sleeve or damage of the foam elastic roller itself will occur.